Manufacture of aromatic sulfonic acids



MANUFACTURE F AROMATIC SULFONIC ACIDS Richard F. Brooks, Webster Groves,Mo., assignor to Monsanto Chemical Company, St. Louis, Mo., acorporation of Delaware Application December 6, 1956 Serial No. 626,594

4 Claims. (Cl. 260-505) No Drawing.

This invention relates to the sulfonation of aromatic hydrocarbons andmore particularly pertains to the manufacture of monosulfonic acids ofaromatic hydrocarbons.

Aromatic sulfonic acids are manufactured in large quantitles asintermediates in the synthesis of phenolic compounds and asintermediates in the synthesis of other organic chemical compounds. Inthe past, aromatic hydrocarbons such as benzene, and naphthalene,toluene, xylene and other lower alkyl substituted benzenes as well asthe alkyl substituted benzenes having long alkyl chains such as kerylbenzene, dodecyl benzene and the like have been sulfonated to thecorresponding monosulfonic acids by reacting the hydrocarbon in theliquid phase with a sulfonating agent such as sulfuric acid, oleum orsulfur trioxide. However, sulfonation of benzene and such other aromatichydrocarbons as the lower alkyl substituted benzenes having 1 to 4carbon atoms in the alkyl groups with sulfur trioxide as with the use ofoleums is accompanied by the objectionable formation of large quantitiesof aromatic sulfones.

The formation of aromatic sulfones is objectionable since the sulfonesrepresent the consumption of a sizeable portion of the aromatichydrocarbon reactant. Furthermore, in the synthesis of phenols, theformation of phenyl sulfone, tolyl sulfone, xylyl sulfone and naphthylsulfones not only consumes two moles of the aromatic hydrocarbon permole of sulfone formed but the sulfone does not react with caustic toform a phenate. The sulfones also add an undesirable property to thesulfonic acid product which would carry over in the synthesis ofderivatives of the sulfonic acid.

In a liquid phase sulfonation of aromatic hydrocarbons with sulfurtrioxide the quantity of sulfones formed can be as high as 30%, that is,the amount of sulfone formed represents the consumption of 30% of thearomatic hydrocarbon raw material. This is especially true in thesulfonation of benzene and toluene with sulfur trioxide for the sulfoneformation can go as high as 30% and about 25% respectively. In thesulfonation of xylene the sulfone formation is not quite as high,attaining a maximum of about 8 to 10%.

Various methods have been proposed for the reduction of objectionableformation of sulfones in the reaction of an aromatic hydr carbon withsulfur trioxide. it has been proposed, for example, to carry out thesulfonation reaction in the presence of a solvent such as chloroform orliquid sulfur dioxide. It has also been proposed to use a sulfurtrioxide complex such as a complex with trioxane as a sulfonating agent.It has been proposed that various sulfone inhibitors be employed. Assulfone inhibitors there have been suggested alkali metal and alkalineearth metal sulfates, sodium sulfate apparently being the preferredinhibitor of this class. It has also been proposed that alkali metal andmkaline earth metal sulfonates be employed as the sulfone inhibitor. Assulfone inhibitors it has also been proposed to use lower saturatedalkyl acids, especially those containing 2 to 3 carbon atoms. The amountof sulfone inhibitor suggested as satisfactory Patented June 2, .1959

according to these prior processes has been about 10% or less by weightbased on the weight of the aromatic compound to be sulfonated. However,as has been pointed out the use of a greater amount of the sulfoneinhibitor is not deleterious to the sulfonation reaction except in someinstances where the sulfone inhibitor itself becomes sulfonated and inthese cases the use of larger quantities of inhibitor will consume largeportions of $0 in sulfonating the inhibiting material. The use of largequantities of other inhibiting material such as the alkali metal oralkaline earth metal sulfonates while not having a deleterious effect onthe sulfonation step does, of course, present a problem of removal ofsaid amount from the reaction medium. If not removed, the excessquantities of the sulfonates would be objectionable, as for example, inthe fusion step in the synthesis of phenol.

It has now been discovered that pyridine added to the sulfonationreaction in an amount equal to about 2% to about 30% of the aromatichydrocarbon charged will suppress the formation of sulfones in thesulfur trioxide sulfonation of said aromatic hydrocarbon.

More specifically, the process of this invention consists in reactingsulfur trioxide with a liquid aromatic hydrocarbon in the presence of 2%to 30% of pyridine at a reaction temperature as heretofore used, underabout 120 C., but preferably the reaction temperature being in the rangeof about 70 C. to about 90 C. There can be employed as the source ofsulfur trioxide, oleum having an 80;, content of 20% and above up to andincluding 100% 80;; with no sulfuric acid being present. In the processof this invention there should be used at least about 2% by weight ofpyridine based on the weight of the aromatic compound to be sulfonated.However, the use of greater than 30% by weight offers no advantage inthe suppression of the formation of sulfones. Hence, as a practicallimitation it has been found that the upper limit on the amount ofpyridine is about 30% by weight.

The following specific examples further illustrate the process of thisinvention.

Example I To a suitable sulfonating apparatus there is added 282 partsby weight'or" a mixture containing 76.5% benzene sulfonic acid, 17.9%sulfuric acid, 4.5% water and 1.1% sulfone, this mixture having beenprepared by a previous monosulfonation of benzene with oleum of about30% 80;. The mixture containing benzene sulfonic acid is stirred andheated to about 80 C. and there are added about 47 parts by weigl t ofsulfur trioxide. Thereafter 89 parts by weight of benzene and 89 partsby weight of sulfur trioxide are added simultaneously at equal weightrates while the reaction temperature is maintained between and 79 C.After all of the reactants are added the resulting mixture is stirredand maintained at 80 to C. for about one hour. From an analysis of theresulting reaction mixture and from the composition of the startingmaterial it is determined that about 18% of the benzene charged isconverted to sulfones.

In contrast to the foregoing process in which no sulfone inhibitor isemployed, the following examples employing the process of this inventionillustrate the results obtained by the use of varying amounts ofpyridine.

Example II To suitable sulfonation apparatus there is charged 234 partsby weight of the benzene sulfonic acid mixture described in Example 1containing 76.5% benzene sulfonic acid, 17.9% sulfuric acid, 4.5% waterand 1.1% suifone, this mixture having been prepared by a previousmonosulfonation of benzene in the presence of a sulfone inhibitor whichhad been removed. There is added to this mixture 9 parts by weight ofpyridine, the resulting mixture heated to about 80 (3., about 47 partsby weight of sulfur trioxide added and thereafter about 89 parts byweight of each of benzene and sulfur trioxide added at equal weightrates over a period of about minutes. Thereafter the reaction mixture ismaintained at a temperature of from 80 to 85 C. for about an hour. Ananalysis of the resulting mixture shows that it contains 73.8% benzenesulfonic acid, 21.1% sulfuric acid, 0.6% water, 1.7% sulfones, 1.9%pyridine and 0.9% benzene. From the composition of the starting mixtureand the mixture resulting from this reaction it is determined that 4.4%of the benzene employed is converted to sulfones.

Example Ill The process of Example II is repeated except that 282 partsof the starting mixture and 24.9 parts by weight of pyridine areemployed. Also about 50 parts by weight of sulfur trioxide are addedbefore the remaining 110 parts by weight thereof and 110 parts by weightof henzene are added substantially simultaneously in equal weight rates.The reaction temperature during the addition of the reactants ismaintained at about to 81 C. and held at to 85 C. for about one hourafter all the reactants have been combined. An analysis of the resultingreaction mixture shows that there is present 72.4% benzene sulfonicacid, 21.3% sulfuric acid, 0.3% water, 0.9% sulfone, 4.1% pyridine andabout 1% benzene. From the composition of the starting mixture and thecomposition of the reaction mixture produced by this process it isdetermined that 0.9% of the benzene charged has been converted to thesulfones.

A comparison of the results of the process of this invention, asillustrated in Examples II and III with the results produced by theprocess of Example I where no sulfone inhibitor was employed,demonstrates how the use of a small amount of pyridine reduces theexcessive formation of sulfones in the monosulfonation of benzene. In asimilar manner there can be suppressed the sulfone formation which wouldotherwise occur during the nuclear monosulfonation of an aromatichydrocarbon such as toluene, naphthalene and other alkyl substitutedbenzenes as well as naphthalene and the like.

It is realized that the complex formed by pyridine with sulfur trioxidehas been suggested as a sulfonating agent. However, it has been foundthat the pyridine complex with sulfur trioxide is not a satisfactorynuclear sulfonating agent for aromatic hydrocarbons and especiallybenzene and alkylated benzenes. Although the pyridine complex withsulfur trioxide may form in the process ofthis invention it is notemployed as a sulfonating agent, but rather the presence of pyridineeither alone or as the complex with sulfur trioxide appears to onlyfunction as a sulfone inhibitor. This is clearly illustrated by theprocess of Example III in which the amount of pyridine employed wasabout 22% by weight of the benzene charged. Amounts of pyridine greaterthan the 22% employed in Example III can be employed. However, theamount of reduction in the sulfone formation will be decreased veryslightly with the use of such greater amounts of pyridine. Thus, for thepractical application of the process of this invention, the amount ofpyridine employed should not be above about 30% by weight of the totalamount of benzene employed.

What is claimed is:

1. A method of preparing monosulfonic acids of aromatic hydrocarbons bythe nuclear sulfonation of a benzene ring which comprises reactingsulfur trioxide with an aromatic hydrocarbon in the liquid phase and inthe presence of from about 2% to about 30% by weight of pyridine basedon the weight of said aromatic hydrocarbon.

2. A method of preparing lower alkyl benzene monosulfonic acids by thenuclear sulfonation of the benzene ring which comprises reacting analkyl benzene in a liquid phase with sulfur trioxide in the presence offrom about 2% to about 30% by weight of pyridine based on the weight ofsaid alkyl benzene.

3. A method of preparing benzene monosulfonic acid which comprisesreacting liquid benzene and sulfur trioxide in the presence of fromabout 2% to about 30% by weight of pyridine based on the weight of saidbenzene.

4. A method of preparing benzene monosulfonic acid which comprisesreacting at a temperature below about C. benzene and sulfur trioxide inthe presence of from about 2% to about 30% by weight of pyridine basedon the weight of said benzene.

OTHER REFERENCES Gilbert in The Chemistry of Petroleum Hydrocarbons,edited by Brooks, pages 611-641, vol. 3 (particularly pages 612-617,620-633, 20 pages), 1955.

1. A METHOD OF PREPARING MONOSULFONIC ACIDS OF AROMATIC HYDROCARBON BYTHE NUCLEAR SULFONATION OF A BENZENE RING WHICH COMPRISES REACTINGSULFUR TROIXIDE WITH AN AROMATIC HYDROCARBON IN THE LIQUID PHASE AND INTHE PRESENCE OF FROM ABOUT 2% TO ABOUT 30% BY WEIGHT OF PYRIDINE BASEDON THE WEIGHT OF SAID AROMATIC HYDROCARBON.